Motor vehicles such as dump trucks, refuse trucks, transit concrete mixing trucks, open-bed trucks, utility trucks, military trucks and other vehicles of various types to which a significant load may be added for transport are limited in their load transporting ability by various factors. Such as the weight bearing capacity of their supporting axles and applicable federal and state law. With such laws for example limiting the gross vehicle weight to 80,000 pounds, the weight carried by a single axle to 20,000 pounds and there being an exception as to consecutive axles that limits them to carrying a prescribed combined weight depending on their number and spacing. For example, the combined axle carrying weight is limited to 34,000 pounds in regard to the two powered tandem axles that are typically employed with heavy duty load-transporting motor vehicles. And with such factors as a result impacting the use of such motor vehicles in that the more weight the vehicle can transport at a time, the more useful the vehicle can be provided other factors that impact the ability of the vehicle to perform in an acceptable manner are also taken into account. With such factors including the axle manufacturers rated load capacity.
And in regard to such motor vehicles where it is desired to increase their load carrying capacity within the prescribed limits, it is common practice to provide one or more auxiliary axles that are deployed on command to help support the motor vehicle and thereby reduce the weight carried by the axles that normally support the vehicle. Wherein the axles that normally support the vehicle are then referred to as primary axles and include at least one axle with steerable wheels and one or more powered axles with wheels. And the auxiliary axles are referred to as either a pusher axle, tag axle or trailing axle in distinguishing between where and how they are employed to help support the motor vehicle. Wherein a pusher axle is suspended from the motor vehicle chassis in a location to operate between a forwardly-located primary axle with steerable wheels under the control of the vehicle operator and one or more rearwardly-located primary axles with powered wheels. Whereas a tag axle is suspended from the motor vehicle chassis to operate rearward of one or more primary axles with powered wheels but not normally at any substantial distance behind the motor vehicle chassis. And a trailing axle (that has also been referred to as a tag axle and trailing tag axle) is also suspended from the motor vehicle chassis but in a manner to operate at a substantial distance rearward of the motor vehicle chassis.
And in considering the use of such auxiliary axles, their individual weight bearing capacity and their number available in being suitably applied to a particular motor vehicle limits their ability to provide the vehicle support desired. And in the case of trailing axles, their number has been limited for the lack of a suitable suspension system that can accommodate more than one to operate in a satisfactory manner at a substantial distance behind the motor vehicle chassis.